As known in the art, many programmable integrated circuits use antifuses as their programmable elements.
Programmable integrated circuits are semiconductor devices which provide an assortment of internal logic functions along with programmable elements that allow users to select the particular logic functions they desire. Antifuses are one such programmable element.
Programmable integrated circuits are fabricated on semiconductor wafers. A typical 6 inch semiconductor wafer contains hundreds of programmable integrated circuit die and the typical integrated circuit contains thousands of antifuses. Thus there are typically millions of antifuses on a single 6 inch semiconductor wafer.
An antifuse uses a layer of dielectric material to achieve its programmable characteristic. As manufactured, a layer of dielectric material gives the antifuse a very high impedance. One dielectric material often used is hydrogenated amorphous silicon (a-Si:H). An antifuse is programmed by applying a voltage of appropriate magnitude and duration to the antifuse. During programming, the antifuse undergoes permanent physical change, the programming voltage breaks down the high impedance dielectric in the antifuse, which results in an element with a much lower impedance. The thickness of the amorphous silicon is a significant factor in determining the magnitude of the voltage needed to program an antifuse. Because the manufacturing process is not perfectly uniform across the entire wafer, some antifuses demonstrate higher or lower programming voltages than other antifuses. It is believed that variations in the amorphous silicon dielectric layer across the wafer is one reason for this variation in antifuse programming voltages. The range of programming voltages demonstrated by the antifuses is called the programming voltage distribution.
Manufacturers of antifuse programmable logic arrays have been seeking antifuse structures with narrower programming voltage distributions so as to achieve higher yield and greater long term reliability. Yield represents the percentage of manufactured components which initially meet the specifications for the product. Reliability represents the integrity of the components over time, i.e. whether the components continue to the meet specifications over long term use. High reliability requires that antifuses maintain high impedance when not programmed. A narrow programming voltage distribution also helps to meet this requirement. Due to the volume of antifuses in a semiconductor, an antifuse structure or process which provides even a modest improvement in the yield or reliability of individual antifuses can represent a substantial breakthrough in the yield or reliability when measured at the device level.